diff --git a/youtube_dl/postprocessor/ffmpeg.py b/youtube_dl/postprocessor/ffmpeg.py
index b21dafba9..c7ebd664f 100644
--- a/youtube_dl/postprocessor/ffmpeg.py
+++ b/youtube_dl/postprocessor/ffmpeg.py
@@ -433,7 +433,9 @@ class FFmpegEmbedSubtitlePP(FFmpegPostProcessor):
 class FFmpegMetadataPP(FFmpegPostProcessor):
     def __init__(self, downloader=None, preferredinfo=None):
         super(FFmpegMetadataPP, self).__init__(downloader)
-        self._preferredinfo = preferredinfo if isinstance(preferredinfo, dict) else None
+        if not (isinstance(preferredinfo, dict) or preferredinfo == None):
+            raise TypeError('preferredinfo must be a dictionary, if provided')
+        self._preferredinfo = preferredinfo
 
     def run(self, info):
         metadata = {}